Contention Groups for Hidden Nodes

ABSTRACT

Communicating among stations in a network includes, from each of multiple stations in the network, transmitting information indicating which other stations from which that station is able to reliably receive transmissions. A schedule for communicating among the stations is determined based on the information from the stations and transmitting the schedule over the network. The schedule includes a plurality of time slots during which respective sets of stations are assigned to communicate using a contention-based protocol.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 60/941,949, entitled “MANAGING COMMUNICATIONS OVER A SHARED MEDIUM,” filed on Jun. 4, 2007, which is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to managing contention groups for hidden nodes.

BACKGROUND

A network of communication stations can share a communication medium (e.g., wires connecting multiple stations or spectrum for transmitting radio signals among stations) using any of a variety of access techniques. Some access techniques (e.g., carrier sense multiple access (CSMA) techniques) include a contention period in which stations contend for use of the medium for transmitting a signal by sensing when the medium is idle. In CSMA techniques, “collisions” sometimes occur when signals from two or more stations overlap. Some CSMA techniques attempt to detect collisions and abort transmission to reduce the negative impact of collisions (e.g., CSMA/CD techniques). Other CSMA techniques include mechanisms to avoid or reduce the probability of collisions (e.g., CSMA/CA techniques). For example, different transmissions may be assigned one of multiple priorities. Access is granted using a Priority Resolution Period in which stations signal the priority at which they intend to transmit, and only the highest priority transmissions are allowed to continue in the contention process. A random backoff mechanism spreads the time over which stations attempt to transmit, thereby reducing the probability of collision.

SUMMARY

In one aspect, in general, a method for communicating among stations in a network includes, from each of multiple stations in the network, transmitting information indicating which other stations from which that station is able to reliably receive transmissions. The method includes determining a schedule for communicating among the stations based on the information from the stations and transmitting the schedule over the network. The schedule includes a plurality of time slots during which respective sets of stations are assigned to communicate using a contention-based protocol.

Aspects can include one or more of the following features.

Each station in a given set is able to reliably receive transmissions from all the other stations in the given set.

The contention-based protocol does not use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.

At least some of the sets include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set.

The contention-based protocol used by at least some of the sets that include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.

At least some of the stations are not able to reliably receive transmissions from at least some of the other stations in the network.

Each station in a given set is able to reliably receive transmissions from all the other stations in the given set.

Each station in a first set of stations assigned to communicate in a given time slot is not able to reliably receive transmissions from any of the stations in a second set of stations assigned to communicate in the given time slot.

The contention-based protocol does not use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.

At least some of the sets include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set.

The contention-based protocol used by at least some of the sets that include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.

The information indicating which other stations from which a station is able to reliably receive transmissions is transmitted to a first station.

The first station determines the schedule.

The first station assigns stations to sets based on the information.

The method further comprises providing the stations in different sets one or more parameters associated with the contention-based protocol independently.

The one or more parameters include at least one of a contention window size and a defer counter.

The one or more parameters provided to a given set include at least one parameter that is selected based on the number of stations in the given set.

The one or more parameters provided to a given set include a random backoff parameter.

The contention-based protocol comprises a carrier sense multiple access (CSMA) protocol.

The CSMA protocol comprises a carrier sense multiple access with collision avoidance (CSMA/CA) protocol.

In another aspect, in general, a method for communicating among stations in a network includes assigning a first station to a first level; assigning stations not assigned to a preceding level that can reliably receive transmissions from a preceding level to a higher level; assigning stations to sets based on the assigned level; and determining a schedule for communicating among the stations and transmitting the schedule over the network. The schedule includes a plurality of time slots during which respective sets of stations are assigned to communicate using a contention-based protocol.

Aspects can include one or more of the following features.

At least some of the stations are not able to reliably receive transmissions from at least some of the other stations in the network.

Each station in a given set is able to reliably receive transmissions from all the other stations in the given set.

Each station in a first set of stations assigned to communicate in a given time slot is not able to reliably receive transmissions from any of the stations in a second set of stations assigned to communicate in the given time slot.

The contention-based protocol does not use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.

At least some of the sets include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set.

The contention-based protocol used by at least some of the sets that include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.

In another aspect, in general, a system for communicating among stations in a network includes a first set of multiple stations assigned to communicate during a first time slot using a contention-based protocol; a second set of multiple stations assigned to communicate during a second time slot using a contention-based protocol; and a third set of multiple stations assigned to communicate during the first time slot using a contention-based protocol. At least one station in the first set is able to reliably communicate with at least one station in the second set, and none of the stations in the first set is able to reliably communicate with any of the stations in the third set.

Aspects can include one or more of the following features.

Each of multiple stations in the network is configured to transmit information indicating which other stations from which that station is able to reliably receive transmissions.

At least one station in the network is configured to determine a schedule for communicating among the stations based on the information from the stations and to transmit the schedule over the network.

The schedule includes the first and second time slots.

Among the many advantages of the invention (some of which may be achieved only in some of its various aspects and implementations) are the following.

The approaches described can improve network efficiency. For example, inefficiencies due to hidden nodes can be reduced by reducing or eliminating hidden nodes, and by trading off the number of contention groups and the reduction in hidden nodes. Contention groups can be assigned to time slots such that network communication resources can be reused efficiently. Beacon transmissions can be used to simplify assignment of contention groups and facilitate distributed protocols for forming the groups.

Other aspects and advantages will be apparent from the detailed description, drawings, appendices and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a communication network.

FIG. 2 is a block diagram of a communication system for communicating over the network.

FIGS. 3A and 3B are timing diagrams of a TDMA scheduling period.

FIGS. 4A-4C and FIG. 6 are schematic diagrams of stations assigned to contention groups and time slot groups.

FIG. 5 is a diagram of stations grouped into beacon levels.

DETAILED DESCRIPTION

There are a great many possible implementations of the invention, too many to describe herein. Some possible implementations that are presently preferred are described below. It cannot be emphasized too strongly, however, that these are descriptions of implementations of the invention, and not descriptions of the invention, which is not limited to the detailed implementations described in this section but is described in broader terms in the claims.

FIG. 1 shows an exemplary network configuration for an access network 100 such as a Broadband Power Line Network (BPLN) that provides access to a backhaul network. A BPLN can be managed by a service provider entity having access to the underlying physical power line medium. A BPLN is a general purpose network that can be used for several types of applications including, smart grid management, broadband internet access, voice and video delivery services, etc. In various implementations, a BPLN can be deployed on low voltage, medium voltage and high voltage power lines. Additionally, a BPLN can span an entire neighborhood or it may be deployed within a single multi-dwelling unit. For example, it can be used to provide network service to tenants in a single apartment building. While power lines are one medium for deploying the BPLN, similar techniques can be deployed on other wire lines, such as, for example, coaxial cables, twisted pair or a combination thereof.

A BPLN can include one or more Cells. A cell is a group of broadband power line (BPL) devices in a BPLN that have similar characteristics such as association management, security, Quality of Service (QoS) and channel access settings, for example. Cells in a BPLN are logically isolated from each other, and communication to and from the backhaul occurs within the cell. Each cell in a BPLN includes a Core-Cell and may also include one or more Sub-Cells. There can be more than one cell on a given physical power line medium.

A Core-Cell includes a group of devices in a BPLN that can share certain functionality such as a common security protocol. An exemplary Core-Cell includes a Head End (HE), Repeaters (R), and Network Termination Units (NTUs), but may exclude Customer Premise Equipment (CPE). The Head End (HE) is a device that bridges a cell to the backhaul network. At a given time, a cell will have one active Head End and the Head End manages the cell including the Core-Cell and any associated Sub-Cells. A Repeater (RP) is a device that selectively retransmits MSDUs to extend the effective range and bandwidth of the BPLN Cell. Repeaters can also perform routing and QoS functions. The NTU is a device that connects a BPLN cell to the end users' network or devices. The NTU may in some cases bridge to other network technologies such as WiFi. A single NTU can serve more than one customer. Each Sub-Cell is associated with an active NTU. In some implementations, an HE, an NTU and/or an RP can be co-located at a single station. Thus, a single device may be designed to perform multiple functions. For example, a single device can simultaneously be programmed to perform the tasks associated with an RP and an NTU.

Various types of CPE devices (e.g., a computer) can be used as endpoint nodes in the network and such devices can communicate with other nodes in the network through the NTU, any number of repeaters, (e.g., including no repeaters), and the Head End. Each node in the network communicates as a communication “station” using a PHY layer protocol that is used by the nodes to send transmissions to any other stations that are close enough to successfully receive the transmissions. Stations that cannot directly communicate with each other use one or more repeater stations to communicate with each other. The stations have the potential to interfere with each other, but techniques can be used to coordinate in a centralized and/or distributed manner.

Any of a variety of communication system architectures can be used to implement the portion of the network interface module that converts data to and from a signal waveform that is transmitted over the communication medium. An application running on a station provides and receives data to and from the network interface module in segments. A “MAC Service Data Unit” (MSDU) is a segment of information received by the MAC layer. The MAC layer can process the received MSDUs and prepares them to generate “MAC protocol data units” (MPDUs). An MPDU is a segment of information including a header (e.g., with management and overhead information) and payload fields that the MAC layer has asked the PHY layer to transport. An MPDU can have any of a variety of formats based on the type of data being transmitted. A “PHY Protocol Data Unit (PPDU)” refers to the modulated signal waveform representing an MPDU that is transmitted over the power line by the physical layer.

Apart from generating MPDUs from MSDUs, the MAC layer can provide several functions including channel access control, providing the required QoS for the MSDUs, retransmission of corrupt information, routing and repeating. Channel access control enables stations to share the powerline medium. Several types of channel access control mechanisms like carrier sense multiple access with collision avoidance (CSMA/CA), centralized Time Division Multiple Access (TDMA), distributed TDMA, token based channel access, etc., can be used by the MAC. Similarly, a variety of retransmission mechanism can also be used. The Physical layer (PHY) can also use a variety of techniques to enable reliable and efficient transmission over the transmission medium (power line, coax, twisted pair etc). Various modulation techniques like Orthogonal Frequency Division Multiplexing (OFDM), Wavelet modulations can be used. Forward error correction (FEC) code like Viterbi codes, Reed-Solomon codes, concatenated code, turbo codes, low density parity check code, etc., can be employed by the PHY to overcome errors.

One implementation of the PHY layers is to use OFDM modulation. In OFDM modulation, data are transmitted in the form of OFDM “symbols.” Each symbol has a predetermined time duration or symbol time T_(s). Each symbol is generated from a superposition of N sinusoidal carrier waveforms that are orthogonal to each other and form the OFDM carriers. Each carrier has a peak frequency f_(i) and a phase Φ_(i) measured from the beginning of the symbol. For each of these mutually orthogonal carriers, a whole number of periods of the sinusoidal waveform is contained within the symbol time T_(s). Equivalently, each carrier frequency is an integral multiple of a frequency interval Δ_(f)=1/T_(s). The phases Φ_(i) and amplitudes A_(i) of the carrier waveforms can be independently selected (according to an appropriate modulation scheme) without affecting the orthogonality of the resulting modulated waveforms. The carriers occupy a frequency range between frequencies f_(i) and f_(N) referred to as the OFDM bandwidth.

Referring to FIG. 2, a communication system 200 includes a transmitter 202 for transmitting a signal (e.g., a sequence of OFDM symbols) over a communication medium 204 to a receiver 206. The transmitter 202 and receiver 206 can both be incorporated into a network interface module at each station. The communication medium 204 can represent a path from one device to another over the power line network.

At the transmitter 202, modules implementing the PHY layer receive an MPDU from the MAC layer. The MPDU is sent to an encoder module 220 to perform processing such as scrambling, error correction coding and interleaving.

The encoded data is fed into a mapping module 222 that takes groups of data bits (e.g., 1, 2, 3, 4, 6, 8, or 10 bits), depending on the constellation used for the current symbol (e.g., a BPSK, QPSK, 8-QAM, 16-QAM constellation), and maps the data value represented by those bits onto the corresponding amplitudes of in-phase (I) and quadrature-phase (Q) components of a carrier waveform of the current symbol. This results in each data value being associated with a corresponding complex number C_(i)=A_(i) exp(jΦ_(i)) whose real part corresponds to the I component and whose imaginary part corresponds to the Q component of a carrier with peak frequency f_(i). Alternatively, any appropriate modulation scheme that associates data values to modulated carrier waveforms can be used.

The mapping module 222 also determines which of the carrier frequencies f₁, . . . , f_(N) within the OFDM bandwidth are used by the system 200 to transmit information. For example, some carriers that are experiencing fades can be avoided, and no information is transmitted on those carriers. Instead, the mapping module 222 uses coherent BPSK modulated with a binary value from the Pseudo Noise (PN) sequence for that carrier. For some carriers (e.g., a carrier i=10) that correspond to restricted bands (e.g., an amateur radio band) on a medium 204 that may radiate power no energy is transmitted on those carriers (e.g., A₁₀=0). The mapping module 222 also determines the type of modulation to be used on each of the carriers (or “tones”) according to a “tone map.” The tone map can be a default tone map, or a customized tone map determined by the receiving station, as described in more detail below.

An inverse discrete Fourier transform (IDFT) module 224 performs the modulation of the resulting set of N complex numbers (some of which may be zero for unused carriers) determined by the mapping module 222 onto N orthogonal carrier waveforms having peak frequencies f₁, . . . , f_(N). The modulated carriers are combined by IDFT module 224 to form a discrete time symbol waveform S(n) (for a sampling rate f_(R)), which can be written as

$\begin{matrix} {{S(n)} = {\sum\limits_{i = 1}^{N}{A_{i}{\exp \left\lbrack {j\left( {{2\; \pi \; \; {n/N}} + \Phi_{i}} \right)} \right\rbrack}}}} & {{Eq}.\mspace{14mu} (1)} \end{matrix}$

where the time index n goes from 1 to N, Ai is the amplitude and Φ_(i) is the phase of the carrier with peak frequency f_(i)=(i/N)f_(R), and j=√−1. In some implementations, the discrete Fourier transform corresponds to a fast Fourier transform (FFT) in which N is a power of 2.

A post-processing module 226 combines a sequence of consecutive (potentially overlapping) symbols into a “symbol set” that can be transmitted as a continuous block over the communication medium 204. The post-processing module 226 prepends a preamble to the symbol set that can be used for automatic gain control (AGC) and symbol timing synchronization. To mitigate intersymbol and intercarrier interference (e.g., due to imperfections in the system 200 and/or the communication medium 204) the post-processing module 226 can extend each symbol with a cyclic prefix that is a copy of the last part of the symbol. The post-processing module 226 can also perform other functions such as applying a pulse shaping window to subsets of symbols within the symbol set (e.g., using a raised cosine window or other type of pulse shaping window) and overlapping the symbol subsets.

An Analog Front End (AFE) module 228 couples an analog signal containing a continuous-time (e.g., low-pass filtered) version of the symbol set to the communication medium 204. The effect of the transmission of the continuous-time version of the waveform S(t) over the communication medium 204 can be represented by convolution with a function g(τ;t) representing an impulse response of transmission over the communication medium. The communication medium 204 may add noise n(t), which may be random noise and/or narrowband noise emitted by a jammer.

At the receiver 206, modules implementing the PHY layer receive a signal from the communication medium 204 and generate an MPDU for the MAC layer. An AFE module 230 operates in conjunction with an Automatic Gain Control (AGC) module 232 and a time synchronization module 234 to provide sampled signal data and timing information to a discrete Fourier transform (DFT) module 236.

After removing the cyclic prefix, the receiver 206 feeds the sampled discrete-time symbols into DFT module 236 to extract the sequence of N complex numbers representing the encoded data values (by performing an N-point DFT). Demodulator/Decoder module 238 maps the complex numbers onto the corresponding bit sequences and performs the appropriate decoding of the bits (including de-interleaving and descrambling).

Any of the modules of the communication system 200 including modules in the transmitter 202 or receiver 206 can be implemented in hardware, software, or a combination of hardware and software.

Various stations in a network may generate regular beacon transmissions for various purposes. A beacon transmission (or simply a “beacon”) includes management information that can be used for a variety of purposes. The stations may communicate with each other in time periods between beacon transmissions, provided the power line channel characteristics between any two communicating stations permit it.

In some networks one of the functions of a beacon transmission is to carry medium allocation (or scheduling) information. The scheduling information allocates some of the time between beacon transmissions as a contention period during which stations may contend for access to the power line medium. The scheduling information also allocates a contention-free period during which times slots are assigned to particular stations for access to the power line medium. The scheduling information is provided relative to a TDMA Scheduling Period Start Time (or TDMA Period Start Time).

The TDMA Period start time is synchronized with respect to the AC line cycle of the power line waveform such that the time between consecutive TDMA period start times is based on the underlying AC line cycle frequency. Thus, to the extent that the AC line cycle frequency may vary, the TDMA period start time (and hence the duration of each TDMA Period) may not be perfectly periodic. Since the beacons transmitted by the HE may not be heard by every station, each station transmits its own beacon to relay the information in the beacon to stations that do not hear the HE. While the stations may transmit their beacons at different times, the TDMA period start time is established with respect to the information contained in the beacons transmitted by the HE. At each station, the TDMA period start time can be synchronized to that of the HE using a start time synchronization procedure described in more detail below. Each station can then predict a given TDMA period end time (or future TDMA period start time) based on the synchronized start time and the local AC line cycle at the given station by correlating the synchronized start time to a detectable feature of the power line waveform such as a zero crossing. The TDMA period can be set by the HE to any multiple of a half of the AC line cycle period, for example, by waiting for a given number of zero crossings.

In some cases it is desirable to increase the TDMA period to make more efficient use of the medium by reducing the percentage of time devoted to sending the “overhead” information in the beacon transmission. There is also overhead information associated with transmissions from the stations during each TDMA period. It may also be desirable to keep the TDMA period small enough to provide a desired number of transmission opportunities in a given length of time to reduce the latency. Thus, the TDMA period can be selected according to a trade-off between keeping overhead low and latency between transmission opportunities low. For example, in some implementations the TDMA period is selected to be twice the AC line cycle period. In this case, when operating in power line environments with an AC line cycle frequency of 60 Hz, the TDMA period would be approximately 33.33 msec. When operating in power line environments with an AC line cycle frequency of 50 Hz, the TDMA period would be approximately 40 msec. Variations in the TDMA period may occur due to drift in the AC line cycle frequency. The HE determines the duration of the TDMA period as well as the start time of the TDMA period.

FIG. 3A shows the structure of an exemplary TDMA period 300 which consists of a Contention Period 302 followed by a Stay-out Period 304 and a Contention Free Period 306. In general, a TDMA period can contain any number of Contention Periods, Stay-out Periods and Contention Free Periods in any order. The TDMA period may also be different for different stations in the BPLN. The Contention Period 302 is a time in which stations can contend for permission to transmit using a shared medium access protocol such as CSMA/CA. The Stay-out Period 304 is a time during which stations are not allowed to transmit. The Contention Free Period 306 includes time slots assigned for use by predetermined stations (e.g., using a Time Domain Multiple Access (TDMA) protocol). Beacons are typically transmitted during the Contention Period. Beacon may also be transmitted during Contention Free Periods. The frequency of beacon transmission depends on how frequently the associated management information needs to be communicated. Typically, beacons are transmitted once in several TDMA periods.

In networks in which repeaters are used to extend the reach of signals in the network, multiple stations can retransmit beacon transmissions to be heard by stations that would otherwise not be able to receive the beacon transmissions from the HE.

In some implementations, contention occurs over all the stations in the network contending during the contention period 302. However, for large networks, even with the use of random backoff intervals, collisions are more likely to occur (e.g., due to multiple layers of hidden nodes). In some implementations, the stations can be broken into groups that perform contention independently (e.g., in different contention periods) without interfering with each other. Each contention group has a smaller number of stations, and can be selected to reduce or eliminate hidden nodes within each group.

For example, the stations can be assigned to “contention groups” centrally by the HE station based on information collected from the stations about which stations can reliably receive transmissions from which other stations, or in a distributed manner as described in more detail below. Within each contention group, a contention-based protocol (e.g., a CSMA protocol) can be used. Different (potentially overlapping) time slots can be allocated (e.g., as determined by the HE) in which contention-based communication occurs based on knowledge of traffic conditions and of routing and network topology. These contention groups can reduce inefficiencies associated with using a contention-based protocol over the entire network that result, for example, from having multiple layers of hidden stations. If hidden stations exist in a group of contending stations, to avoid collisions, Request To Send/Clear To Send (RTS/CTS) signals can be used. However, these RTS/CTS have associated overhead (e.g., time used to send the signals) that reduce efficiency. In some implementations, stations are assigned to contention groups so that each contention group does not have any hidden stations (i.e., such that all the stations in a given contention group can hear each other). Without hidden stations among the stations in a contention group, the need for using RTS/CTS signals can be eliminated, improving efficiency for each group. In some implementations, as described in more detail below, stations are assigned to contention groups to reduce the layers of hidden stations, while still allowing some hidden nodes to exists in order to reduce the number of contention groups.

Different contention groups can also be assigned different time slots so that stations in neighboring contention groups do not interfere. For example, the time slots can be assigned during the Contention Free Period 306 to ensure that contention occurs between stations assigned to the same contention group without interference from stations assigned to different contention groups. One or more contention groups that do not interfere (e.g., if the contention groups are far enough apart) and are assigned to the same time slot are called a “time slot group.” Time slot groups facilitate channel reuse, as explained in more detail below. The contention groups also enable customizing parameters to each contention group based on characteristics of the group, such as the number of stations in the group. Different backoff parameters, for example, can be used. Other parameters that can be customized for each contention group are a contention window size and a defer counter.

FIG. 4A shows an example, of an assignment of contention groups and time slot groups in a case in which there is a linear chain of stations including an HE station and repeater stations R1, R2, R3, R4, etc. In FIG. 4A, stations that can hear each other are connected with a line. Thus, HE can hear R1, R1 can hear HE, and R2, and so on. Each station is assigned a position in the chain such that the HE station is in position 0, repeater station R1 is in position 1, repeater station R2 is in position 3, and so on. In this example, in order to eliminate hidden nodes within contention groups and ensure that no contention groups interfere with each other, only two time slots are needed. The stations are divided into contention groups of two and each contention group is assigned to one of two time slot groups (Group A and Group B), starting with HE and R1 in time slot Group A, R2 and R3 in time slot Group B, and R4 and R5 in time slot Group A, and so on (alternating assigning adjacent contention groups to different time slot groups). All stations with a position equal to 4n or 4n+1 (where n=0, 1, 2, . . . ) in time slot Group A contend during a first time slot, and all stations with a position equal 4n+2 or 4n+3 (where n=0, 1, 2, . . . ) in time slot Group B contend during a second time slot that does not overlap with the first time slot. The start and end times of the time slots for each time slot group can be indicated in a message sent by the HE station and passed on to all stations in the network. FIG. 3B shows the time slot 316A used as a contention period for time slot group A and time slot 316B used as a contention period for time slot group B.

In some network topologies, complete elimination of hidden nodes can result in large number of contention groups. Since more contention groups generally call for more time slots for contention, this reduces the network efficiency. Thus, there is a trade off between the selection of a larger number of contention groups without hidden nodes and a smaller number of contention groups with hidden nodes. In the latter case, a smaller number of contention groups will result in lower efficiency loss due to grouping, however, the need to handle hidden nodes will result in efficiency loss due to collisions and use of RTS/CTS.

FIG. 4B and FIG. 4C show different groupings of contention groups and assignment to time slot groups for the same network topology. Contention groups are shown with dashed ovals and time slot groups are labeled. In FIG. 4B, the grouping is such that there are no hidden nodes in any of the contention groups. Three separate time slots (for three respective time slot groups) are required to schedule all of these groups so that transmissions by stations in one group do not interfere with the transmissions by stations in another group. FIG. 4C shows another grouping in which only two time slots (for two respective time slot groups) are required to schedule them. However, the single contention group assigned to time slot group B has hidden nodes, and the collisions and the RTS/CTS mechanisms used to address this hidden node problem will result in some efficiency loss. The grouping used in FIG. 4C provides better network efficiency than the grouping used in FIG. 4B. The grouping used in FIG. 4C requires only two time slots, which results in more channel reuse than that used in FIG. 4B.

In some implementations, the HE station can gather information about which stations can hear which other stations and management messages from the HE tell the stations which group they are assigned to. In some implementations, the stations can perform a distributed protocol, to determine which stations can hear each other and select contention groups such that the stations in a given group can hear each other.

Each station in the BPLN broadcasts a beacon transmission regularly but not necessarily at the same time (or at the same frequency) at which the HE broadcasts beacon transmissions. For example, a Repeater station may transmit its beacon within a given target maximum delay (e.g., 100 ms) after reception of the beacon it is tracking. The beacon transmission can contain many fields including a Beacon Level (BL) field. The BL is used to determine the hierarchy of stations in the network. The HE is considered to be a Beacon Level 0 station and it sets the BL field in the beacon transmission to be zero. All the stations that can reliably receive (or “hear”) the beacon transmissions sent by the HE (with BL=0) are Beacon Level 1 stations and set the BL field in their beacon transmissions to 1. For all other stations, the BL is one higher than the smallest BL of all the stations it can reliably hear. FIG. 5 shows an example of stations grouped into Beacon Levels. A group of stations labeled HE and S1-S6 may be a head end and a group of repeaters in a BPLN, for example. The stations are shown arranged in a graph with a line between stations that represents the ability to reliably receive a beacon transmission. In this example, stations S1 and S2 are Beacon Level 1 stations and S3, S4, S5 and S6 are Beacon Level 2 stations.

Beacon Level information can be used by stations to determine the contention groups and time slot groups in a distributed manner. For example, contention groups include stations of two consecutive beacon levels, and stations whose beacon levels are {4n, 4n+1} (where n=0, 1, 2, . . . ) belong to one time slot group and stations whose beacon levels are {4n+2,4n+3} (where n=0, 1, 2, . . . ) will belong to a second time slot group. This grouping will result in a schedule with two time slots. Another grouping could result in requiring three time slots with stations whose beacon levels are {6n,6n+1} (where n=0, 1, 2, . . . ) belong to a time slot group scheduled in the first time slot, those with beacon levels {6n+2,6n+3}(where n=0, 1, 2, . . . ) belong to a time slot group scheduled in the second time slot and, those with beacon levels {6n+4,6n+5}(where n=0, 1, 2, . . . ) belong to a time slot group scheduled in the third time slot. FIG. 6 shows a network with the beacon level of each station shown next to it. Using the beacon level grouping mechanism and considering a schedule of two time slots, stations with beacon levels 0, 1, 4, 5 belong to time slot Group 1 and stations with beacon levels 2, 3 belong to time slot Group 2. Hence stations HE, R1, R5, R6, R7 and R8 belong to time slot Group 1 and stations R2 and R3 belong to time slot Group 2.

Many other implementations of the invention other than those described above are within the invention, which is defined by the following claims. 

1. A method for communicating among stations in a network, the method comprising: from each of multiple stations in the network, transmitting information indicating which other stations from which that station is able to reliably receive transmissions; determining a schedule for communicating among the stations based on the information from the stations and transmitting the schedule over the network; wherein the schedule includes a plurality of time slots during which respective sets of stations are assigned to communicate using a contention-based protocol.
 2. The method of claim 1, wherein each station in a given set is able to reliably receive transmissions from all the other stations in the given set.
 3. The method of claim 2, wherein the contention-based protocol does not use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.
 4. The method of claim 1, wherein at least some of the sets include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set.
 5. The method of claim 4, wherein the contention-based protocol used by at least some of the sets that include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.
 6. The method of claim 1, wherein at least some of the stations are not able to reliably receive transmissions from at least some of the other stations in the network.
 7. The method of claim 6, wherein each station in a given set is able to reliably receive transmissions from all the other stations in the given set.
 8. The method of claim 7, wherein each station in a first set of stations assigned to communicate in a given time slot is not able to reliably receive transmissions from any of the stations in a second set of stations assigned to communicate in the given time slot.
 9. The method of claim 7, wherein the contention-based protocol does not use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.
 10. The method of claim 6, wherein at least some of the sets include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set.
 11. The method of claim 10, wherein the contention-based protocol used by at least some of the sets that include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.
 12. The method of claim 1, wherein the information indicating which other stations from which a station is able to reliably receive transmissions is transmitted to a first station.
 13. The method of claim 12, wherein the first station determines the schedule.
 14. The method of claim 10, wherein the first station assigns stations to sets based on the information.
 15. The method of claim 1, further comprising providing the stations in different sets one or more parameters associated with the contention-based protocol independently.
 16. The method of claim 15, wherein the one or more parameters include at least one of a contention window size and a defer counter.
 17. The method of claim 15, wherein the one or more parameters provided to a given set include at least one parameter that is selected based on the number of stations in the given set.
 18. The method of claim 17, wherein the one or more parameters provided to a given set include a random backoff parameter.
 19. The method of claim 1, wherein the contention-based protocol comprises a carrier sense multiple access (CSMA) protocol.
 20. The method of claim 19, wherein the CSMA protocol comprises a carrier sense multiple access with collision avoidance (CSMA/CA) protocol.
 21. A method for communicating among stations in a network, the method comprising: assigning a first station to a first level; assigning stations not assigned to a preceding level that can reliably receive transmissions from a preceding level to a higher level; assigning stations to sets based on the assigned level; and determining a schedule for communicating among the stations and transmitting the schedule over the network; wherein the schedule includes a plurality of time slots during which respective sets of stations are assigned to communicate using a contention-based protocol.
 22. The method of claim 21, wherein at least some of the stations are not able to reliably receive transmissions from at least some of the other stations in the network.
 23. The method of claim 22, wherein each station in a given set is able to reliably receive transmissions from all the other stations in the given set.
 24. The method of claim 23, wherein each station in a first set of stations assigned to communicate in a given time slot is not able to reliably receive transmissions from any of the stations in a second set of stations assigned to communicate in the given time slot.
 25. The method of claim 23, wherein the contention-based protocol does not use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.
 26. The method of claim 22, wherein at least some of the sets include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set.
 27. The method of claim 26, wherein the contention-based protocol used by at least some of the sets that include one or more stations that are not able to reliably receive transmissions from at least some other stations in the same set use Request to Send/Clear To Send (RTS/CTS) transmissions among the stations in a given set.
 28. A system for communicating among stations in a network, the system comprising: a first set of multiple stations assigned to communicate during a first time slot using a contention-based protocol; a second set of multiple stations assigned to communicate during a second time slot using a contention-based protocol; and a third set of multiple stations assigned to communicate during the first time slot using a contention-based protocol; wherein at least one station in the first set is able to reliably communicate with at least one station in the second set, and none of the stations in the first set is able to reliably communicate with any of the stations in the third set.
 29. The system of claim 28, wherein each of multiple stations in the network is configured to transmit information indicating which other stations from which that station is able to reliably receive transmissions.
 30. The system of claim 29, wherein at least one station in the network is configured to determine a schedule for communicating among the stations based on the information from the stations and to transmit the schedule over the network.
 31. The system of claim 30, wherein the schedule includes the first and second time slots. 